1. Field of the Invention
The present invention relates to hemicyanine dyes and optical recording media formed using a hemicyanine dye as an optical recording material, and more particularly, to hemicyanine dyes, and a write only read many (WORM) optical recording medium capable of recording and reproducing information by laser, which employs a hemicyanine dye as an optical recording material.
2. Description of the Related Art
Optical recording media have a smaller recording area per recording unit than conventional magnetic recording media, so that use of optical recording media as a high density recording medium is extensively increasing. Optical recording media are classified into a read only memory (ROM) type for only reading recorded information, a write once read many (WORM) type which allows only one writing, and an erasable type which allows erasing of recorded information and rewriting.
Recordable optical recording media record and reproduce information by detecting variations in the reflectivity caused by physical deformation and changes in phase and magnetic properties of a recording layer before and after the recording.
A compact disk recordable (CD-R) is widely known as a WORM type optical recording medium. In addition, a variety of optical recording materials, which ensure easy manufacture of such a recording medium with improved properties, have been suggested and some of them are in use. Disclosed optical recording materials for CD-Rs include a cyanine dye (Japanese Patent Publication No. sho 58-125246), a phthalocyanine dye (European Patent No. 676,751), an azo dye (U.S. Pat. No. 5,441,844), a dye of double salt (U.S. Pat. No. 4,626,490) and a dye of azo-metal complex (U.S. Pat. No. 5,272,047).
Recently, with increased amount of information, a digital versatile disk recordable (DVD-R) with increased information storing capability has been suggested. The DVD-R adopts a red diode laser having a wavelength of 630-690 nm, as a light source, and has reduced pit size and track interval, so that its information storing capacity is 6-8 times increased to CD-Rs, with improved recording density. Optical recording materials suitable for DVD-Rs needs a high reflectivity and good absorption at the recording wavelength, and good solubility in an organic solvent. In addition, the synthesis of optical recording materials must be easy with low costs. Examples of such optical recording materials for DVD-Rs include a cyanine dye (Japanese Patent Publication Nos. hei 10-149583 and hei 9-208560) and an azo-metal complex (Japanese Patent Publication Nos. hei 10-157293 and hei 9-157301). However, it would be desirable to improve the characteristics of the conventional dyes.
To solve the above problems, it is a first objective of the present invention to provide a novel hemicyanine dye which is easy to synthesize with high yield.
A second objective of the present invention is to provide an optical recording medium with improved recording characteristics, which is manufactured using the hemicyanine dye as an inexpensive optical recording material.
The first objective of the present invention is achieved by a hemicyanine dye having formula (1): 
wherein Z is a benzene ring, naphthalene ring or anthracene ring; X1 is S, O, Se, NR or C(CH3)2; X2 is S, NR or O, where R is hydrogen or an alkyl group of 1 to 5 carbon atoms; R1 is hydrogen, halogen atoms, an alkyl group of 1 to 3 carbon atoms or a nitro group; each of R2 and R5 is an alkyl group of 1 to 5 carbon atoms; each of R3 and R4 is hydrogen, an hydroxy group, an alkyl group of 1 to 4 carbon atoms, halogen atoms or a halide; Y is Cl, Br, I, ClO4, BF4, BrO4, PF6, CH3SO3, CF3SO3, 4-CH3C6H4SO3, C6H5SO3, 6-SO3C10H6SO3 or HSO4; n is an integer of 0-2; and m is an integer of 1-2.
The second objective of the present invention is achieved by an optical recording medium comprising a transparent substrate with grooves, a recording layer including a laser absorptive dye, formed over the transparent substrate, and a reflective layer and a protective layer formed over the recording layer, wherein the dye of the recording layer is a hemicyanine dye having formula (1): 
wherein Z is a benzene ring, naphthalene ring or anthracene ring; X1 is S, O, Se, NR or C(CH3)2; X2 is S, NR of O, where R is hydrogen or an alkyl group of 1 to 5 carbon atoms; R1 is hydrogen, halogen atoms, an alkyl group of 1 to 3 carbon atoms or a nitro group; each of R2 and R5 is an alkyl group of 1 to 5 carbon atoms; each of R3 and R4 is hydrogen, an hydroxy group, an alkyl group of 1 to 4 carbon atoms, halogen atoms or a halide; Y is Cl, Br, I, ClO4, BF4, BrO4, PF6, CH3SO3, CF3SO3, 4-CH3C6H4SO3, C6H5SO3, 6-SO3C10H6SO3 or HSO4; n is an integer of 0-2; and m is an integer of 1-2.